This invention relates to a method of monitoring an automation system, having a control unit and a plurality of terminals. In particular, this invention relates to reliable detection of multiple signal changes in a message telegram between a terminal and a control unit in an automation system.
In automation systems having a plurality of terminals (also known as automation equipment) and control units (also known as operator control and monitoring stations or “operator stations”), events are represented in binary signals. Signal changes (level-triggered changes) are detected by the terminals and reported via message telegram to one or more control units, where the reported signal level is displayed and processed further.
Conventionally, a second message, e.g., with the content that the signal has changed again in the meantime, can be sent only after the control unit has confirmed, to the terminal, receipt of the first message. This confirmation is made with an acknowledgment message.
If a single signal is reported in a message telegram, then, conventionally, the last two signal changes are recognized. The boundary conditions for this are: a) an additional information “overflow” is provided in the message telegram, indicating that one or more signal changes could not be reported; b) a message having the same signal status as the signal that was reported last and stored in the control unit has been received.
To increase the efficiency of the signaling protocol and to more effectively utilize system resources, message telegrams containing more than a single signal are used, namely eight signals, for example. The additional information in the form of the “overflow flag” in the message telegram and the fact that a message telegram has been received constitutes a 1:n relationship for a plurality of signals. In this case, it is no longer possible to assign the extra information in the overflow flag and the event “reception of a message telegram” to an individual signal among the plurality of signals. Therefore, this allocation can no longer be used for the generally valid signal tracking. The available information in the message telegram is instead reduced to the particular signal status. An interim change from the original state to a second state and back again is not recognized.